1. Field of the Invention
This invention relates to an optical recording medium on and from which information can be optically recorded and reproduced. It also relates to a process for producing the same.
2. Description of the Prior Art
In recent years, optical disks from which any stored information is read out by utilization of light have been put into practical use. Such optical disks are made for reproducing sounds or animation images recorded in disks, using semiconductor laser beams, and have come into wide use as compact disks (hereinafter "CD") and video disks.
Development is energetically made also on optical disks on and from which information is recorded and reproduced by the irradiation with laser beams to cause changes in optical characteristics (refractive index, extinction coefficient, phases, etc.), what is called writing once optical disks.
Known methods include a method of recording information by irradiating a recording medium with a laser beam to cause melt evaporation so that holes or pits are made in the recording medium, where the recording medium is then continuously irradiated with a laser beam weak enough to cause no changes in the recording medium, to detect the presence of the pits on the basis of the strength of reflected light, and a method of recording information in which an amorphous recording medium with a low reflectance is heated by irradiation with a laser beam to cause phase changes so that the recording medium is brought into a crystalline state with a high reflectance.
Under such circumstances, no optical disk has been put into practical use in regard to an optical disk on and from which users can record information and reproduce the recorded information by means of a commercially available CD player.
The reason therefor is that in order to satisfy the CD standards it is required for the optical disk to have a reflectance through its substrate, of not less than 70% (i.e., to have a reflectance of 70% or more when, for example, a semiconductor laser beam with a wavelength of 780 nm is made incident on the optical disk from its readout side), and also to satisfy the conditions that a sufficient modulation degree can be achieved at a recording linear velocity of 1.2 to 1.4 m/s and that tracking error signals on a required level must be obtained.
In particular, the requirements that the information can be recorded using outputs from a semiconductor laser and yet the high reflectance must be achieved conflict with each other. Great efforts have been made so that this problem can be solved.
Incidentally, there is a recent progress in the practical utilization of colorant optical disks that have a lower thermal conductivity and higher recording sensitivity than metal type materials, have superior stability and also have the possibility of a cost decrease such that they can be manufactured using a relatively inexpensive apparatus. Such colorant optical disks, however, have some disadvantages, one of which is that they have a low reflectance.
Takeda, Shinohara, Kusakawa and Hirohashi, "JOHO KIROKU SISUTEMU ZAIRYO (Materials for Information Recording Systems)", Gakujutsu Shuppan Center, 1989, disclose that the reflectance of a colorant thin film can be improved by making large the refractive index n (the real part of a complex refractive index) of a colorant and making small the extinction coefficient k (the imaginary part of a complex refractive index), whereby the reflectance can be increased up to about 40%. They also report that a reflective film comprised of gold (Au) may be provided on a colorant coating film to make it possible to reproduce information by the use of commercially available CD players.
Now, in order to satisfy the CD standard that the reflectance is not less than 70% and to ensure the light absorbance at which it becomes possible to carry out recording, it is necessary to provide a reflective film comprised of Au, Ag, Pt or the like, having a high reflectance at wavelengths pertaining to semiconductor lasers. For example, Japanese Laid-open Patent Application No. 2-42652 discloses a writing once optical disk comprising a transparent substrate provided with tracking guide grooves, and a lamination of a recording film containing a special colorant and a reflective film comprised of Au having a high reflectance. This optical disk can attain the reflectance as prescribed in the CD standard, i.e. a reflectance of 70% to 90% against the light with wavelength of 780 nm at the land of the surface on which signals are recorded.
Such noble metals, however, result in a high cost, which is a disadvantage for achieving wide spread of optical disks. On the other hand, when an inexpensive metal such as aluminum or an aluminum is used for the reflective film, it is difficult to make up optical disks having the reflectance of 70% to 90%.
In order to increase the reflectance of the colorant itself, the colorant must be synthesized with newly designed molecular structure.
Moreover, although the reflectance can be increased when an inorganic interference layer having a high refractive index is provided between a colorant layer and a reflective layer, this requires film formation by vacuum deposition. The film formation by vacuum deposition, however, necessarily requires a large-scale apparatus and a long-term operation including the operation for evacuation carried out over a period of several hours, resulting in a high cost.